Oriented films with a first outer layer of a polyester or copolyester, a second outer layer of a polyolefin, and a gas-barrier layer of EVOH have been described in the patent literature, see for instance EP-A476,836, WO 99/55528, WO 99/44824, WO 99/44823, EP-A-1,190,847, and WO 01/98081.
In all those cases, the structures containing a first outer polyester layer, a second outer polyolefin layer, and a core EVOH layer also contain a core polyamide or i polyester layer.
In particular, EP-A-476,836 describes an oriented laminated film with a surface layer of a polyester, an EVOH core layer, an intermediate layer of certain polyamides and a heat-sealing layer of polyolefin. In the structures there claimed a given thickness ratio between the outer polyester layer and the core polyamide layer needs to be present, what improves orientability of the tape. The films described there are said to have excellent stretching processability, heat-sealing and packaging properties and good transparency after heat-sterilization.
WO 99/44824 and WO 99/44823 describe EVOH-containing heat-shrinkable films with at least four layers, i.e. a first outer layer comprising i.a. a polyethylene, a second outer layer possibly comprising a polyester, a core EVOH layer and an additional core polyamide or polyester layer. The bags obtained therefrom can be stacked on top of one another and sealed simultaneously and the presence of the core polyamide or polyester layer is said to give i.a. enhanced impact strength and render the tape more easily orientable.
WO 99/55528, EP-A-1,190,847, and WO 01/98081 relate to heat-shrinkable structures where in addition to the polyester and polyolefin outer layers, a core polyamide layer is always present and optionally also an EVOH core layer. These films are said to satisfy various properties required of a packaging material, through e.g. a control of the heat-shrinkage stress and of the heat-shrink.
In all the above documents the process actually described for the manufacture of these films is the so-called trapped bubble process. According to this technique, the polymer feeds are extruded through an annular die to give a thick tubing, called “tape”. Said tubing is quickly quenched at the exit of the extrusion die in order to control crystallization, then it is re-heated to the suitably selected orientation temperature and oriented transversely by inflating it with a gas to expand its diameter and longitudinally by running the nip rolls that hold the bubble at a differential speed.
It has now been found that it is possible to obtain biaxially oriented films with a first outer layer of a polyester or copolyester, a second outer layer comprising an ethylene-or propylene-homo-or co-polymer, and a core layer comprising EVOH, without needing any polyamide or polyester core layer, by carrying out the biaxial orientation of the extruded tape by means of a tenter frame, preferably a simultaneous tenter frame.
It has been found that the films which are thus obtained have a high modulus in at least one direction, and are therefore very useful for most of the currently used packaging systems as it is known that for a good machinability, as well as for a good printability, the packaging material needs to be stiff, i.e. it should have a high modulus.
The films of the present invention are in particular characterized by a modulus which is higher than 6,000 kg/cm2 in at least one direction.
Preferred films according to the present invention have a modulus which is higher than 6,500 kg/cm2 in at least one direction, and more preferred are those films which have a modulus higher than 7,000 kg/cm2 in at least one direction.
It has also been found that when a heat-shrinkable structure is obtained, it is possible to conjugate a high free shrink with a low shrink force, particularly in the transverse direction. This would be an advantage in all the packaging applications where the product to be packaged is sensitive to a high shrink force and in particular can be crushed or distorted by films with a high shrink force when these films are shrunk around the product.
Preferred heat-shrinkable films according to the present invention have in fact a free shrink of at least 10% in each direction at 120° C. and a maximum shrink tension in the transverse direction, in the temperature range of from 20 to 180° C. of less than 5 kg/cm2, more preferably less than 3 kg/cm2, and even more preferably less than 1 kg/cm2. 